A big quake could leave Wellington cut off from the rest of the North Island for four months, new disaster predictions show.

It could take 40 days to restore the water supply to even a basic level, while road access could take up to 120 days, according to "worst case" predictions presented to the region's Civil Defence Emergency Management Group.

That could leave Wellington residents or commuters trapped in the city for months, and dependent on water rations being distributed by authorities for about six weeks.

The predictions relate to a quake of magnitude 7.5 or higher, and come after a magnitude-7 quake off Taranaki on Tuesday.

The long, rolling quake, at 230 kilometres deep, was felt throughout the lower North Island and upper South Island at 10.36pm. Rail services were suspended while tracks were inspected for damage.

The time frames for recovery are in a report presented to the Emergency Management Group last week which looks at how long it will take to restore "lifeline" services – road access, water and power.

Wellington's hilly topography means it will take longer to restore services to a "survival level" than it did in a flatter city such as Christchurch.

Water would be restored more quickly in Petone and the Hutt Valley. For water, survival level is defined as a limited supply that requires boiling.

Before that, water would have to be distributed from reservoirs and storage sites – probably rationed to 20 litres a person a day – and there would be no road access or power.

After the Christchurch quake in February last year, it took about two weeks to restore water to about 85 per cent of homes, but "boil water" notices remained in place until April.

Survival level road access is defined as one lane with speed restrictions. The Manawatu Gorge slip, which closed the state highway for months, shows how long large slips can take to clear, the report says.

The time frames are "indicative only", it says, and regional Emergency Management Group co-ordinator Bruce Pepperell stressed they were for the worst-case scenario.

But understanding how long it could take to restore services meant strategies to hasten those processes could be devised, he said.

"You need to look at that in a lot more depth ... We can put in place some plans to overcome some of this."

Access was an important difference between Wellington and Christchurch. Wellington had just two, both of which were regularly affected by slips.

The time lines highlighted the importance of preparation and having good systems in place – as was emphasised by Tuesday's quake, Mr Pepperell said.

"While it was a really significant shake, it was deep, so it didn't have the same twist in the tail that we sometimes get ... It is a timely reminder that we do live on the shaky isles – so to speak – and we need to be prepared."

Civil Defence Emergency Management Group chairwoman and Kapiti Coast Mayor Jenny Rowan said the "single most important message" from the report findings was to prepare.

People who lived in outer areas of the region and travelled to central Wellington for work could be cut off for a long time. "You could be in a situation where you're needing to look after yourself ... You could be trapped in that city with no way of getting out other than by foot."

Although it caused only minor damage – cracked chimneys and stock falling from supermarket shelves – Pepperell said the earthquake was a "timely reminder" to residents, businesses and organisations to update their disaster response strategy.

Mr Pepperell said there had been a "flurry of activity" by councils yesterday, which had used the tremor as a cue to reassess their disaster response readiness.

"Hopefully this earthquake will enable people to wake up and review their own personal and business circumstances," he said.

The issue of a tsunami warning was tricky, Mr Pepperell said. "If you live on the coast and you get a particularly nasty earthquake that knocks your socks off your feet, my recommendation is get more height."

The risk of a tsunami was low on Tuesday because the quake struck so deep.

Civil Defence is preparing for its September 26, 9.26am earthquake drill, hoping to get one million people to "drop, cover and hold".

The regional office, which absorbs the work provided by the region's nine councils, was launched on Monday.

The departing former head of Wellington City's emergency management, Fred Mecoy, had one parting message for Wellingtonians: "Don't stop being prepared – you can never be prepared enough."